1. Field of the Invention
The present invention relates to power management for electronic devices, and more particularly to measuring battery capacity.
2. Description of the Related Art
Currently, to meet users' various demands, more and more functionalities are integrated into portable devices. However, the power source has quickly become a bottleneck to such functionality expansion. One concern is that improvement of battery power density cannot keep up with the demands, and another concern is that the dynamic load profile makes tracking battery status a complex task. Usually, depletion of the battery power without timely alert can lead to unpleasant results, and to preclude the unpleasant results, it is desirable for users to be informed of the battery status. Now, more and more circuit designers are casting their eyes on battery monitor apparatuses, fox example, battery gas gauge, to improve capacity measurement and prediction of battery run-time.
One gas gauge approach is to use voltage measurement for battery capacity reports, since batteries always exhibit certain voltage versus capacity characteristics. Although simple and low in hardware cost, the voltage-based scheme compromises the accuracy by failing to account for the elusive battery impedance. Therefore, the voltage measurement is always affected by battery impedance effect, which can make the voltage measurement invalid for capacity prediction.
An alternative to the voltage-based scheme is to count coulomb charge passing in and out of batteries via an accumulation counter in addition to the voltage measurement. Coulomb charge, computed by integrating battery current over time, is a representation of battery capacity. The gas gauges based on coulomb charge counting and voltage measurement schemes have two types, with and without a CPU. The gas gauge with a CPU provides a stand-alone solution for battery capacity reports. All measurement and computation are accomplished by the battery gas gauge, and key battery parameters, such as remaining capacity and relative state of charge, are directly accessible by the host through the communication port. The gas gauge without a CPU, known as a battery monitor, provides coulomb data to the host along with battery voltage and temperature readings. The host develops a gas gauge code to process the data and compute the remaining capacity of the battery. Unfortunately, the gas gauge code is unfamiliar to engineers specializing in software development at the host end, and it is usually a complicated task to compute the remaining capacity of the battery at the host. Further, though the gas gauge with the CPU can directly provide the host with the remaining capacity of the battery, the financial cost of this approach is higher.
Regardless whether equipped with a CPU, one concern with the coulomb counting approach is that the initial capacity is estimated using the simple voltage-based measurement which induces inaccuracy at the beginning of the coulomb counting. Another concern with the coulomb counting approach is that during the dynamic monitor stage, accumulated error and offset will inevitably occur and result in inaccuracy for a long term monitoring. Therefore, it is desirable to have a system and method which can enhance the measurement accuracy without additional circuitries and it is to such a system and method the present invention is primary directed.